Having known a few women who were in the business, the credibility issue doesn't come from their being "bad", but because of their immersion in a life where truth is an illusion.
After years of instinctively telling people what you think they want to hear, and years hanging out with people who spin stories to make themselves look good (or to harm others who they don't like) the concept of "objective truth", or even "objective reality" tends to fade and fantasy becomes real.
In rape cases one is usually reduced to "he said, she said" as "evidence." It is obvious that both parties may have excellent reasons for lying.
When witness testimony is in direct conflict, the jury must decide which person is telling the truth. If the witnesses' character and history is not taken into account, only gender and skill at telling the story can be used to decide between alternate versions of what happened.
Some people are excellent liars. Often such a person can tell the story more believably than someone who is actually telling the truth.
The truth such women live are a function of the manipulation and cruelties (both physical and psychological) their pimps inflict on them, up to an including threatening their families. Many of them live in fear.